A polyphenylene sulfide (hereinafter, sometimes abbreviated to PPS) resin is superior in characteristics such as heat resistance, chemical resistance, flame retardancy and an electric insulating property, and is suitably used as engineering plastics, films, fibers and non-woven fabrics. Particularly, the non-woven fabric made of PPS fibers is expected to be used for industrial applications such as heat-resistant filters, electric insulating materials and separators for a cell using these characteristics.
As a method for producing a non-woven fabric made of PPS fibers, a short fiber non-woven fabric made of staple fibers is proposed (refer to Japanese Patent No. 2764911). However, to obtain a non-woven fabric, it is necessary that a PPS resin is melted and spun into threads, and the threads are bound into a tow form, wet-drawn in a separate step, heat treated under tension, provided with crimping, cut into a staple, and further processed by a mechanical entangle-bonding apparatus such as a card machine or a needle punch in another step. Therefore, this method requires many steps.
A simple production method by the so-called spunbonding method, in which a PPS resin is spun and drawn by an ejector, and directly formed into a filament non-woven fabric, is proposed as a countermeasure for these problems. Specifically, a filament non-woven fabric, which is obtained by spinning a PPS resin by a spunbonding method to form a fabric, stretching the fabric at a glass transition point or higher, preferably biaxially stretching the stretched fabric, and embossing the resulting fabric, is proposed (refer to Japanese Unexamined Patent Publication No. 2005-154919). Moreover, a method for producing a filament non-woven fabric, in which a fabric obtained by spinning a PPS resin by a spunbonding method is temporarily bonded at a first crystallization temperature or lower, and then heat treated at the first crystallization temperature or higher under tension, and permanently bonded, is proposed (refer to Japanese Unexamined Patent Publication No. 2008-223209). The heat treatment under tension in JP '209 aims at promoting the crystallization of a PPS resin, which is hardly achieved only by the steps of spinning and drawing, to realize low shrinkage and dimensional stability. JP '209 shows that if the heat treatment under tension is not performed prior to the bonding or if the heat treatment under tension is insufficient, a problem of irregular width shrinkage due to heat contraction arises in this bonding step.
That is, to obtain a non-woven fabric formed by embossing a non-woven web made of a PPS resin hitherto obtained by a spunbonding method and thermobonding the resulting spunbonded web, stretching treatment under heating or heat treatment under tension is required as a preceding step of thermobonding of a non-woven web or fabric. This method has disadvantages that facilities for drawing a non-woven web or fabric under heating or heat treating a non-woven web or fabric under tension are required, which results in a complicated step with multiple stages, and that energy consumption is large, which results in cost increase, compared with a production method for preparing a spunbonded non-woven fabric using a general-purpose resin such as polyester or polypropylene, and is not necessarily a preferable method.
Further, there is also a proposal that the need for the facilities for heat treatment is eliminated by improving dimensional stability in the step of spinning a PPS resin. For example, there is a method in which dimensional stability against heat is improved by copolymerizing the PPS resin with trichlorobenzene, and spinning and drawing the resulting copolymer as an improvement means based on raw materials (refer to Japanese Patent No. 2890470). However, when the PPS resin is copolymerized with trichlorobenzene, there is a problem that stringiness is deteriorated and thread breakage during spinning and drawing occurs often, and this method lacks production stability.
Further, as an improvement means in the spinning step, a method for producing a heat-resistant non-woven fabric, in which a degree of crystallization of a fiber is improved by spinning a PPS resin at an extremely high spinning speed to suppress heat contraction, is proposed (refer to International Publication WO 2008/035775). However, in this method, since an extremely high spinning speed (7,000 to 11,000 m/min in Examples of WO '775) is employed, and due to which the amount of deformation of the fiber increases, the fiber cannot follow the deformation thereof and thread breakage tends to occur often, and since much compressed air is required, there is a problem that energy consumption is large.
As described above, the current situation is that a method for stably producing, by a simple step, a filament non-woven fabric using a PPS resin, which does not produce irregular width shrinkage, wrinkling, or surface irregularity due to heat contraction, has not been proposed.
Hence, in view of the above-mentioned problems of the prior art, it could be helpful to provide a production method which can obtain a filament non-woven fabric containing PPS as a main component, in which irregular width shrinkage, wrinkling, or surface irregularity due to heat contraction during thermocompression bonding of a non-woven web do not occur, by a simplified step in which stretching treatment under heating and/or heat treatment under tension are not performed on a non-woven web or fabric.